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Shot of alcohol greases micromachines

By Celeste Biever

A tiny shot of alcohol could one day perk up worn-out micro-motors in digital camera displays, car airbags and other micro-electro-mechanical systems (MEMS). The injected alcohol would cause an alcoholic vapour to condense on the tiny crevices in the motor and provide lubrication.

Seong Kim of Pennsylvania State University suggested the idea at the American Chemical Society conference in Anaheim, California, on Monday. He and his colleague Ken Strawhecker showed that methanol, ethanol, propanol and butanol deposited in this way reduced friction between silicon dioxide plates by 80 per cent, while providing a replenishable method for lubrication.

For the parts in a motor to slide past each other, they must overcome adhesion forces, caused by moisture from the air, and friction. In large-scale engines, such as those in cars, oils are used as lubricants.

“But MEMs devices are very small and their parts are very light,” explains Strawhecker. So oils no longer work because their viscous drag is too strong for the tiny machines to overcome.

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Nooks and crannies

To solve the problem, MEMS are commonly coated in solid carbides or nitrides. But these eventually wear out and cannot easily be replaced. If the lubricant is instead deposited as a liquid, surface tension prevents it reaching all the nooks and crannies inside the tiny motors.

However, alcohols can be deposited as a gas, even though they are liquid at room temperature.

Alcohol molecules have another key property – they are oily at one end but water-loving at the other. The water-loving end dissolves in moisture that has accumulated on the surface of the silicon dioxide. This leaves the oily section of the molecule pointing upwards to provide lubrication.

Strawhecker explains that just the right amount of alcohol to form a molecule-thick coating would be injected into a pocket of alcohol-saturated air that encased a micro-motor.

Less drag

Bharat Bhushan, who studies the lubrication of MEMS at Ohio State University, agrees the issue is a crucial one&colon; “With MEMS, we need to reduce the friction and increase the durability.”

He notes that gases have been used to lubricate MEMS before. His work with the semiconductor company Texas Instruments involved the heat-induced deposition of fluorinated organic gases. The method is already used to lubricate the motors that focus light in digital displays, Bhushan says, reducing friction by 50 per cent.

But Strawhecker points out that no heat is required for his method and that the smaller alcohol molecules would have less viscous drag than the molecules used by Texas Instruments, which are over twice as long and so bond to each other more strongly.